1. Field of the Invention
The present invention relates to an apparatus for the production of polycondensation products, such as linear polyesters and co-polyesters. More particularly, the present invention relates to improved reactor internal components designs for use in vertical oriented polymerization reactors.
2. Background Art
Processes for producing polymeric materials such as polyesters and co-polyesters via polycondensation reactions involve the liberation of by-products as the polymeric functional groups of the molecules react with one another to produce longer molecular chain molecules. Typically, the extraction of these liberated by-product molecules from the reaction mixture is necessary in order to drive the molecular build-up of the polymer. If the by-product compounds were not removed, chemical equilibrium will inhibit the length of the formed polymeric chain. In many of these polycondensation reaction systems the preferred method for extracting the liberated by-product is to vaporize the by-product out of the reaction mixture.
Various reactor designs and multi-step reaction systems have been designed and operated to facilitate the vaporization of by-products and the associated production of polycondensation materials. The most economical design for such polycondensation reactions (at least for the production of low to moderate molecular weight polymeric materials) is a series of stirred tank reactors. In these reactor systems large quantities of materials can be produced that use mechanical agitation, thermosiphon reboilers, and/or simple bubble agitation to enhance heat transfer and liquid-vapor surface area renewal. Unfortunately, the viscosity of the polymeric melts increase dramatically as the degree of polymerization (“DP”) increases. Accordingly, because of the practical limitations of agitator designs, the high viscosity of these materials greatly decreases the capability of renewing the liquid-vapor surfaces and hence decreases the mass transfer efficiency of the stirred tank reactor.
In addition to the features set forth above, other operating parameters may be limited in the polycondensation process. For example, higher temperatures may be desirable to increase reaction kinetics and volatility of reaction by-products. Higher volatility of the by-products decreases by-product concentration in the reaction mixture, thereby furthering the polymerization reaction. However, the temperature sensitivity of the polymeric material to degradation reactions limits the use of increasingly higher temperature as a means of furthering the degree of polymerization. Similarly, the volatility of the by-products may be further increased by the use of low operating pressures. However, use of extremely low operating pressures is limited by the cost of achieving low operating pressures and the amount of reactor vapor space needed to prevent entrainment of polymer into the vacuum source. Moreover, the depth of the polymeric pool can inhibit the effective use of the reaction volume in low pressure polycondensation reactors. Specifically, excessive depth of the reaction mixture increases the diffusion and convection paths that volatile by-products must travel before escaping. Furthermore, as the depth of the polymeric pool increases, the deeper portions of the pool are subjected to greater hydrostatic pressure. Higher local pressures within the liquid inhibit the formation of by-product bubbles, which hinders the liberation of the by-products and hence the effective use of the reaction volume for furthering polymerization.
For the reasons set forth above, increasing the degree of polymerization requires replacement of simple stirred tank reactors with specialized reaction equipment. Such specialized equipment must overcome one or more of the operating limitations above to achieve the desired degree of polymerization. Currently, there are two fundamental approaches for enhanced liquid-vapor surface renewal that are best described as the dynamic approach and the static approach.
The first approach might be termed the dynamic approach in that it involves the use of moving mechanical devices to enhance liquid-vapor surface renewal. As noted above, enhanced liquid-vapor surface renewal facilitates the liberation of the by-products. With the dynamic approach, seals are needed around the rotating shaft or shafts that pass through the reactor walls. These seals must be maintained in order to prevent air from leaking into the reactor. Also with the dynamic approach, as the size of the vessel and the viscosity of the product increase, the size of the mechanical components must increase in order to handle the increase in load. The second approach can be referred to as the static approach in that no moving devices are used for the liquid-vapor surface renewal. This later approach uses gravity in combination with vertical drop to create thin polymeric films. Typically, such polymeric films flow between trays during the vertical drop. The thin polymeric films combined with shearing and surface turnover effects created by vertical falling films drive the polymerization reaction by enhancing the liberation of by-products.
Prior art patents which disclose the use of gravity in combination with vertical drop include: U.S. Pat. No. 5,464,590 (the '590 patent), U.S. Pat. No. 5,466,419 (the '419 patent), U.S. Pat. No. 4,196,168 (the '168 patent), U.S. Pat. No. 3,841,836 (the '836 patent), U.S. Pat. No. 3,250,747 (the '747 patent), and U.S. Pat. No. 2,645,607 (the '607 patent). Early tray designs used vertically spaced circular trays (full circle in combination with hollow circle, and segmented circular) that utilized most of the cross-sectional area of the vessel. These circular tray reactors use a large portion of the available pressure vessel's horizontal cross-section for liquid hold-up. In some designs, a circular tray was followed by a hollow circle tray thus forming a disc-and-doughnut arrangement. Thus, polymer flowed over a circular edge as it passed from tray to tray. The liberated gas by-product thus flowed through circular and annular openings. In other designs, the trays were segmented to provide a straight edge for the polymer to flow over before dropping to the next tray. The segmented tray design also provided open area between the straight edge over which the polymer flowed and the vessel wall through which the gas by-product could pass. With both designs however, the vaporized by-products from the trays were forced to flow through the same space as the polymer melt flow. To address this concern, the diameter of the circular trays was made somewhat less than the reactor vessel's diameter. The resulting annular space was used to allow vapor traffic to escape each tray and travel to the reactor vessel's vapor discharge nozzle along a path external to the path of the polymer flow. A shortcoming of the simple circular tray designs is the existence of very slow moving or stagnant regions on the trays. The polymer in these stagnant regions tend to overcook, become excessively viscous, cross-link and/or degrade, and as a result slowly solidify. The net result is a loss of effective reaction volume.
The next generation of designers changed the shape of the trays from circular to other geometric shapes. They eliminated dead zones which are not entirely effective as reaction volume. The elimination of dead zones also improved product quality since the dead zones are regions which produce high levels of degradation products due to the overcooking of the polymer. Unfortunately, these non-circular-shaped trays did not increase the effective use of the cylindrical pressure vessel's cross-sectional area.
The basis for more recent inventions of the '590 patent and the '419 patent is a hollow circular tray which more efficiently utilizes the cross-sectional area of a cylindrical pressure vessel while providing polymer melt flow paths which minimizes liquid dead zone regions and prevent channeling. The net result was an approximate 40% increase in tray area available for liquid retention as compared to the non-circular shaped trays. The central opening in the trays provided a chimney through which the vapor by-products are removed.
However, as set forth above, the depth of the polymeric pools can also inhibit the effective use of the reaction volume at low operating pressures. At a given operating pressure (vacuum level), the negative impact of the deeper polymer depth increases in proportion to the degree of polymerization. This is due to reduction of the chemical equilibrium driving force for polymerization as the concentration of polymer end groups are reduced through the growth of the polymer chains. Hence, to get acceptable results, the mechanisms for liberating polycondensation by-products from the polymer melt must be further enhanced. At higher degrees of polymerization this is necessary so that sufficiently low levels of by-products remain in the melt enabling the polymerization to proceed efficiently. However, another important factor is that viscosity increases substantially as polymerization proceeds to higher degrees of polymerization.
At a sufficiently high viscosity, tray designs which utilize essentially horizontal trays cannot achieve the desired combination of both high polymer throughput and shallow polymer depths. The designs of Lewis et al. in the '168 patent achieve a degree of control over the polymer depth by having the polymer flow down sloping trays. The slopes of the successive trays are increased to account for the expected increasing viscosity of the polymer as it polymerizes along its course. The inventions claimed in the '168 patent are extensions of those sloped tray designs for polymer systems with higher throughputs, even higher viscosities, and/or shallower operating depths.
The design of the '168 patent (roof-and-trough trays) also achieved some degree of control over polymer depth by splitting the polymer flow into two equal streams (with one flow path being a mirror image of the other flow path) that traverse from the top to the bottom of the reactor over sloped trays. The '168 patent design innovation over simple sloped trays was a reduction of the reactor vessel volume needed to enclose the trays within a vacuum environment. By splitting the polymer flow the vertical dimension (vertical drop) needed for a tray to achieve a desired slope and hence a desired polymer depth was reduced. The roof-and-trough configuration cuts the horizontal length of the tray that each half of the polymer flow must traverse before dropping to the next tray. Since each half of the polymer flow traverses half the horizontal distance, the residence time for each is approximately the same as a simple sloped tray while using less total vertical height.
As the production rates are increased, the roof-and-trough design concept can be extended by splitting the polymer streams into more equal streams, generally in binary fashion—two, four, eight . . . Thus, good utilization of the reactor vessel volume is maintained as the vessel increases in size to accommodate the polymer throughput:
However, even with the roof-and-trough tray design of Lewis, utilization of the reactor vessel volume decreases as the desired degree of polymerization is pushed higher and/or the mass transfer versus residence time operating window is narrowed to achieve better quality. As the targeted degree of polymerization is pushed higher, the polymer viscosity increases. Thus, to maintain the same polymer depth requirements steeper tray slopes are required. Similarly, if mass transfer is to be increased by targeting shallow polymer depths, then steeper trays are needed. At some point the slopes become essentially vertical (greater than 60° slope from horizontal) and appreciably thinner depths for a given combination of throughput and viscosity cannot be achieved by further changing the slope. In this region of high throughputs, targeted shallow depths, and high viscosity, the baffle assembly modules of the present invention described herein increase the number of polymer sheets within a given reactor vessel cross-sectional area, thereby achieving high throughputs and better mass transfer.
Accordingly, there is a need for improved tray designs for polycondensation reactors that make more efficient utilization of space in a vertical, gravity flow driven polymerization reactor for combinations of high viscosity, high throughput, and shallow polymer depths.